The present invention relates generally to the laser art, and more specifically concerns apparatus to accomplish retroreflective feedback of certain spectral lines of a generated laser beam back into the gain region of the laser.
A laser will emit light over a known range of frequencies in specific spectral regions referred to as spectral lines. This is a term borrowed from conventional optics and refers to relatively narrow, separate bands or groups of frequencies. These spectral lines define the pattern of the emitted radiation.
In a laser emission spectrum, each spectral line is a band of radiation, concentrated at several specific frequencies within the spectral line band. These specific frequencies are referred to as the modes of a spectral line.
When a laser beam having a spectrum which includes a number of spectral lines is directed through the atomsphere to a particular target, the energy at one or more of the spectral lines is absorbed to some degree by the atmosphere. Certain spectral lines are almost entirely absorbed, while others are reduced to a lesser extent. This phenomenon of atmospheric absorption of particular laser wavelengths directly reduces the amount of energy delivered to the target, and is hence undesirable.
In addition to this direct reduction of laser energy, atmospheric absorption of spectral lines indirectly reduces the amount of energy in the propagated laser beam delivered to the target. The indirect reduction is caused by a phenomenon referred to as thermal blooming, in which the absorbed laser energy causes a heating of the atmosphere, which leads to a decrease in atmospheric density. The index of refraction of the atmosphere changes in accordance with a decrease in density, so a decrease in atmospheric density will lead to a diverging laser beam. This causes a substantial decrease in the intensity of the laser beam at the target. Any decrease in beam intensity at the target is very serious, and must be avoided or decreased as much as possible.
Accordingly, a general object of the present invention is to provide a laser line selection apparatus which overcomes one or more of the disadvantages of the prior art stated above.
It is another object of the present invention to provide such an apparatus which locks a laser onto spectral lines which are not absorbed by the atmosphere.
It is an additional object of the present invention to provide such an apparatus which is capable of selecting any one or several spectral lines within the laser spectrum.
It is a further object of the present invention to provide such an apparatus which does not decrease the original power and/or efficiency of the laser.